1. Field of the Invention
This invention relates to a plasma ashing method used in a semiconductor fabrication process, more particularly to a plasma ashing method for control of generation of residual resist.
2. Description of the Prior Art
FIG. 1A shows mainly a cross sectional view of an inner portion of a quartz chamber in a compositional diagram of a conventional barrel-type plasma ashing device.
FIG. 1B shows a right side view of the plasma ashing device as shown in FIG. 1A. In these drawings, a reference number 21 designates a semiconductor substrate installed on quartz boat 22. A number 24 is a quartz chamber or a ceramics door to close the chamber 23. A reference number 25 denotes a vacuum pump for evacuating the chamber 23 through an exhaust pipe 28. An electrode 26 is located along the external wall of the chamber 23. The electrode 26 is connected to a high-frequency power electric source 29. A reference number 29 is an introduction pipe in order to introduce an ashing gas for a plasma ashing into the chamber 23.
Hereinafter, we will now describe steps of a plasma ashing process by using the plasma ashing device having the composition as described above.
First, the semiconductor substrate 21 on which an organic compound, for instance a photo resist pattern is formed is installed on the quartz boat 22, then placed in the chamber 23.
Next, the ceramic door 24 is closed for hermetically sealing the chamber 23. Then, the air in the chamber 23 is exhausted through the exhaust pipe 28 for keeping the chamber 23 at a predetermined high vacuum. Oxygen gas is provided into the chamber 23 through an introduction pipe 27 until the gas pressure in the chamber 23 reaches a predetermined value.
Next, the high-frequency electric power source 29 is applied to the electrode 26 to generate a plasma in the chamber 23. Thereby, the oxygen gas is excited to become active oxygen. The active oxygen reacts with the organic compound formed on the semiconductor substrate 21.
The reaction process may be expressed by the following equation. EQU C.sub.x H.sub.y +O*=CO.uparw.+H.sub.2 O.uparw.+CO.sub.2 .uparw.(1)
where
the symbol `O*` is the active oxygen and PA1 the symbol `.uparw.` denotes evaporation.
The organic compound on the semiconductor substrate 21 is removed through the above described reaction process and plasma ashing process is accomplished.
In the prior art, the voltage supplied from the high-frequency power source 27 is increased to speed up the reaction process (1). Thereby, the concentration of the active oxygen O* and the temperature in the chamber 23 become high. In addition, the reaction process is rapidly advantaged by directly heating the chamber 23 with a heater.
The condition of the plasma ashing process used in the prior art is as follows:
______________________________________ PLASMA ASHING PROCESS CONDITION Example 1 Example 2 ______________________________________ CHAMBER d .times. L 250 .times. 400 300 .times. 450 [mm] HIGH-FREQUENCY 600 800 APPLIED POWER [W] HIGH-FREQUENCY 0.19 0.19 APPLIED POWER INTERNAL WALL IN CHAMBER [W/cm.sup.2 ] ASHING RATE [.ANG./min] HEATER ON 750 800 HEATER OFF 500 600 ______________________________________
In the conventional plasma ashing process as mentioned above, a bridge between resists is easily produced. The bridge is often left as a resist-residue on the semiconductor substrate 21 after the plasma ashing process is finished.
Moreover, the resist-residue is bonded so strongly on the semiconductor substrate 21 that it cannot be removed easily. It is difficult to remove adequately the resist residue 41 by a wet etching process which is performed later.
In the later processes, abnormality of the resist film deposited around the resist-residue on the substrate occurs.
Thereby, it is difficult to form a flat resist film on the semiconductor substrate. Reliability of the products fabricated by using the conventional method and productivity of the semiconductor device are degreased.
Accordingly, this is one of the factors causing the producer's price of the semiconductor device to be high.